1. Field of the Invention
The field of art to which this invention pertains is distillation apparatus and process including vapor treating devices.
2. Description of the Prior Art
The reclamation of water from human urine has presented aerospace engineering with a challenging task. The systems required on long duration space missions for water recovery of about 220 kg per man per month from crude generated waste water has resulted in many zero gravity system design attempts. Although some credible and efficient design systems have been developed, all have serious design and operational problems. Accordingly, an acceptable system is still being sought.
Raw urine contains approximately 4% by weight of urea and various salts in solution. As water is removed from the urine, solids rapidly concentrate and when, for example, 95% of the original water content of the urine is removed, the residual solids concentration of the remaining fluid is in the order of 30%. And while the requirement for a distillation product from precipitate-forming corrosive fluids is routinely encountered in the chemical industry in a gravity environment, the task of the aerospace engineer is complicated by the fact that a similar task must be accomplished in a zero gravity environment and with severe launch weight, volume and power consumption restrictions imposed.
Three of the more prominent waste recovery systems presently known are "The Air Evaporation Subsystem" (AES), "The Vapor Diffusion Reclamation System" (VDR), and "The Vapor Compression Distillation System" (VCD). In the AES system, electrical energy is used to provide the heat for evaporation of urine contained in a wick. Evaporated "pure" water is carried away by recirculated air, and subsequently condenses downstream of the wick on a conventional plate fin condenser cooled by a low temperature fluid. Wicks are periodically replaced when the solids content builds up and prevents sufficient wick feeding. In the VDR system, electrically heated urine is evaporated through a flat membrane sheet that is held in place by a pressurized diffusion gap between the membrane and condensed on a porous cold plate condenser, also using a low temperature cooling fluid. A recirculated urine-brine solution becomes concentrated as water is evaporated and is contained in a replaceable recirculation tank. While the system does have some advantages, it requires high energy input. The VCD system also uses a recirculation loop and a replaceable brine concentrate tank but employs a rotating drum to generate a gravity field that allows the evaporator and condenser surfaces to function. Steam evaporating over the inner drum surface is compressed and allowed to condense due to its elevated saturation temperature over the backside of the rotating evaporator drum. The elevated saturation temperature provides the potential required for conversion and transfer of the heat of condensation to the heat of evaporation. This is a regenerative system that requires approximately 25% of the energy requirements of the two previous systems. While AES is a relatively simple system which provides for good liquid separation, the problems of wick blockage, efficient wick feeding, and the heating requirements are significant disadvantages. While VDR has the advantages of good liquid separation and bacterial control, the necessity for a flat membrane and its heating requirements are significant problem areas. And while VCD does have the advantage of being a low power system, it has the disadvantages of being complex, noisy, and produces poor liquid separation, especially during shutdown periods.
Accordingly, what has been lacking in the art is a low-power, compact and gravity insensitive distillation system which has a relatively simple design and gives good component separation and bacterial control.